I am trying to setup a GIT repository over LAN in Ubuntu OS.
I am able to setup a GIT repository, but not sure HOW TOexpose the repository to other users in LAN.
As it requires a server, I installed OpenSSH-Server. But don’t know HOW TO configure it.

go into your /etc/ssh/ssh_config file and add git to the list of Allowed Users that can login.
copy your id_rsa.pub file over to your server somewhere (in our example we're using /tmp) and then run this command:

Configure gitosis for a new project. Use your favorite editor to create a new block under the gitosis one. It should look like this:

[group myrailsapp]
members = myNameAsInTheRsa.pub
writable = myNewApp

A couple of things to watch out in the above block.
First, make sure your name matches what's in your public key (that is, open your id_rsa.pub file and see that what the name says.
Second, make sure you spell writable correctly!

Your best way of doing that would be to run a ssh server and limit your users to use git-shell for their login shell. You can install an ssh server properly by either running one of the following commands if it has not already been installed.

sudo apt-get install openssh-server

or

sudo apt-get install dropbear

Then add /usr/bin/git-shell to /etc/shells by doing

sudo echo $(which git-shell) >> /etc/shells

Then use the git-shell instead of bash as your user shell. This will restrict the user to only being able to perform git operations when logging into the server instead of providing the user with a full she..